1. Field of the Invention (Technical Field)
The present invention relates to metrology, and more particularly to scatterometer, ellipsometer and similar analysis methods utilizing angle-of-incidence measurements, and yet more particularly to an apparatus and method that utilizes differentially changing the illumination and observation numerical apertures and determinations based thereupon.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
A variety of scatterometer and related devices and measurements have been used for characterizing the microstructure of microelectronic and optoelectronic semiconductor materials, computer hard disks, optical disks, finely polished optical components, and other materials having lateral dimensions in the range of tens of microns to less than one-tenth micron. For example, the CDS200 Scatterometer, made and sold by Accent Optical Technologies, Inc. is a fully automated nondestructive critical dimension (CD) measurement and cross-section profile analysis system, partially disclosed in U.S. Pat. No. 5,703,692. This device measures the specular reflection of a sample as a function of angle-of-incidence and polarization by scanning a laser beam laterally across the entrance pupil of a high numerical aperture lens system. This device can repeatably resolve critical dimensions of less than 1 nm while simultaneously determining the cross-sectional profile and performing a layer thickness assessment. This device monitors the intensity of a single diffraction order as a function of the angle of incidence of the illuminating light beam. The intensity variation of the 0th or specular order as well as higher diffraction orders from the sample can be monitored in this manner, and this provides information that is useful for determining the properties of the sample target which is illuminated. Because the process used to fabricate the sample target determines the properties of a sample target, the information is also useful as an indirect monitor of the process. This methodology is described in the literature of semiconductor processing. A number of methods and devices for scatterometer analysis are taught, including those set forth in U.S. Pat. Nos. 4,710,642, 5,164,790, 5,241,369, 5,703,692, 5,867,276, 5,889,593, 5,912,741, and 6,100,985.
Scatterometers and related devices can employ a variety of different methods of operation. In one method, a single, known wavelength source is used, and the incident angle {circle around (-)} is varied over a determined continuous range. In another method, a number of laser beam sources are employed, optionally each at a different incident angle {circle around (-)}. In yet another method, an incident broad spectral light source is used, with the incident light illuminated from some range of wavelengths and the incident angle {circle around (-)} optionally held constant. Variable phase light components are also known, utilizing optics and filters to produce a range of incident phases, with a detector for detecting the resulting diffracted phase. It is also possible to employ variable polarization state light components, utilizing optics and filters to vary the light polarization from the S to P components. It is also possible to adjust the incident angle over a range Φ, such that the light or other radiation source rotates about the target area, or alternatively the target is rotated relative to the light or other radiation source. Utilizing any of these various devices, and combinations or permutations thereof, it is possible and known to obtain a diffraction signature for a sample target.
Besides scatterometer devices, there are other devices and methods capable of determining the diffraction signatures at the 0th order or higher diffraction orders using a light-based source that can be reflected off of or transmitted through a target sample, such as a diffraction grating, with the light captured by a detector. These other devices and methods include ellipsometers and reflectometers, in addition to scatterometers.
A number of methods of determining CD utilizing various techniques and devices are disclosed in the prior art. Thus U.S. Pat. No. 5,910,842, to Piwonka-Corle et al., discloses a method and system for spectroscopic ellipsometry employing reflective optics over a range of incident angles. This discloses an actuator-positioned plate with an aperture therein, to observe incidence angles in a selected narrow range. However, this patent does not disclose opening the aperture differentially to integrate a wider range of incident angles.
U.S. Pat. No. 5,877,859, to Aspnes and Opsal, and related U.S. Pat. No. 5,596,411 to Fanton and Opsal, disclose an ellipsometer and method of ellipsometry. Use of an aperture in certain embodiments is disclosed, but only to control the size of the field of the sample that is ultimately imaged on a detector array. In a related approach, an angle-integrated ellipsometer is also provided, but at a fixed range of angles over a large range of wavelengths.
U.S. Pat. No. 5,166,752, to Spanier et al., discloses an ellipsometer and method of ellipsometry. This provides a variable aperture, but does not disclose varying the aperture during a measurement cycle, and further involves separate simultaneous detection of a plurality of different angles of incident light.
One method employed for determination of CD is by measurement of what is called the BRDF (Bi-directional Reflectance Distribution Function). In this method, the BRDF is the fraction of a light beam of wavelength λ and polarization state described by Stokes vector S0 incident on a scattering surface at azimuth angle Φ0 and zenith angle {circle around (-)}0, scattered into a differential solid angle δΩ centered at the azimuth angle Φ and zenith angle {circle around (-)} with polarization state described by Stokes vector S. The BRDF for most surfaces is a complicated function of the incident angle and Stokes vector of the incident photon, the scattering direction, and the surface properties. An ideal mirror, by contrast, has the simplest BRDF: it is unity when the scattered zenith angle is equal to the incident zenith angle and the scattered azimuth angle is opposite the incident azimuth angle, zero for all other scattering angles, and the scattered polarization state is equal to the incident polarization state.
Many surfaces have symmetry properties that reduce the number of independent measurements necessary to describe its BRDF. For example, a diffraction grating of infinite lateral dimensions constructed from dielectric materials does not rotate or mix the polarization state of light incident orthogonal to the grating grooves. The dependence of the BRDF on incident polarization state at this orthogonal orientation is described simply as two independent BRDFs representing the S and P incident polarization states. Under these conditions the BRDF is also symmetric under grating azimuth rotations of 180 degrees. It is typical for commercial scatterometers to take advantage of these symmetries to simplify the BRDF model and the measurement apparatus.
The BRDF is a broadly applicable definition for the quantification of surface scattering, and is most often applied to optically rough surfaces that scatter light broadly into many directions. By contrast, most common optical surfaces do not scatter light broadly; they are optically smooth and have simple mirror-like BRDFs. Such surfaces are typically described by reflectance, diffraction efficiency, or polarization rotation, rather than by stating their full BRDF. These surface properties are, however, special cases of the more general BRDF.
Opaque optical surfaces are well described by their BRDF. However, description of translucent optical surfaces may additionally require description of the Bi-Directional Transmission Distribution Function (BTDF), which is defined for transmission measurements analogously to the BRDF. The BRDF and BTDF are themselves subsets of the Bi-Directional Scattering Distribution Function (BSDF) which describes surface scattering under the most general conditions.
In the semiconductor industry, integrated processing and manufacturing devices are being constructed, incorporating various stepper or exposure components, developing components, baking components, metrology components and the like. Thus a standard silicon wafer of any conventional size is introduced into the integrated processing and manufacturing device, and all fabricating and metrology steps take place within the device, generally under computer control, with a finished wafer, with desired structures etched thereon, exiting the device. This approach preferably requires integrated metrology, such as measurement of CD. It is thus desirable that these components be miniaturized to the extent possible, and occupy the smallest possible footprint, and further have a minimum of moving parts.